Along with the popularization of digital cameras and scanners and the like, users have many opportunities to output and appreciate images printed using printers and to save them, and demand for the ability to output images with favorable colors is increasing.
Color image devices have different gamuts of color reproduction depending on their characteristics. Hence, it is conventionally known to perform conversion (color matching) in an effort to provide uniform color appearance of an image as between different devices.
When an image device B has a broader gamut than that of an image device A, image device B can faithfully reproduce the colors of image device A. However, when an image device D has a gamut narrower than that of an image device C, as in a case wherein a color image displayed on a monitor is to be output by a printer, image device D cannot faithfully reproduce the colors of image device C.
That is, the gamut of equipment such as a monitor and the gamut of another type of equipment such as a printer are different. Also, the gamuts of printers are different in accordance with printing conditions such as printing method, kind of paper, printing materials (inks or toners) and the like.
In order to suppress a drop in color reproducibility due to a difference between input and output gamuts, color matching requires gamut mapping as processing for mapping an input color within an output gamut while maintaining its tint as much as possible.
As gamut mapping, various methods have been proposed.
For example, a method that maps input color within an output gamut to minimize a color difference has been proposed.
This gamut mapping can obtain a satisfactory color matching result in middle and high saturation ranges upon comparing between images before and after mapping. However, this gamut mapping often gives an unnatural impression as a whole. This is because such gamut mapping does not consider differences of white (R, G, B)=(255, 255, 255) and black (R, G, B)=(0, 0, 0) and a gray line that connects white and back of the input and output gamuts, since it attaches importance on only absorption of a to color differences between the input and output gamuts.
In printed matter, the white normally depends on that of paper, and the black depends on the color of a color former such as ink, toner, and the like. In this way, white and black, and a gray line that connects them depend on the characteristics of devices.
FIG. 1 shows a gamut 1001 of a monitor, a gamut 1002 of a printer, a gray line 1003 of the monitor, and a gray line 1004 of the printer on a lightness L*-saturation C* plane of a CIELAB space. FIG. 2 shows gamuts 1011 and 1012, and gray lines 1013 and 1014 of different print sheets in the lightness L*-saturation C* plane of the CIELAB space.
As shown in FIGS. 1 and 2, not only gamuts but also gray lines differ depending on devices to be used or the types of print sheets to be used. Especially, the gray line determines the tint of the entire image. For example, in order to implement color matching between different printers upon, e.g., proofing, color processing that absorbs not only a difference between gamuts but also differences of white, black, and a gray line is required.